The present invention concerns a method for determining a mass flow of ammonia between two SCR catalytic converters disposed one after the other in an exhaust system. Furthermore, the present invention concerns a computer program that implements each step of the method, as well as a machine-readable memory medium that stores the computer program. Finally, the invention concerns an electronic control unit that is arranged to implement the method.
Selective catalytic reduction (SCR) by means of ammonia or ammonia-releasing reagents represents a promising method for reducing nitrogen oxides in oxygen-rich exhaust gases. The efficiency of a SCR catalytic converter depends on the temperature thereof, on the space velocity of the exhaust gas and very decisively on the level of the ammonia absorbed at the surface thereof. Where absorbed ammonia is also available for the reduction of nitrogen oxides in addition to directly introduced ammonia, the efficiency of the SCR catalytic converter is increased compared to an emptied catalytic converter. The storage behavior depends on the respective operating temperature of the catalytic converter. The lower the temperature, the greater is the storage capacity.
If an SCR catalytic converter has completely filled the storage thereof, then in the event of load steps of an internal combustion engine, the exhaust gases of which are being reduced by means of the SCR catalytic converter, ammonia slip can occur even then if no further ammonia or ammonia-releasing reagents are dispensed into the exhaust system. If the highest possible nitrogen oxide conversions are to be achieved, then it is certainly unavoidable to operate the SCR system with a high level of ammonia. If the temperature of the completely filled SCR catalytic converter then increases because of a load step of the internal combustion engine, then the ammonia storage capacity thereof reduces, which causes ammonia slip.
Said effect is particularly pronounced as a result of the fact that SCR catalytic converters are installed close to the internal combustion engine, so that the SCR catalytic converter rapidly reaches the operating temperature thereof following a cold start of the internal combustion engine. A second SCR catalytic converter downstream of the first SCR catalytic converter can therefore be provided in the exhaust system in order to absorb and then convert ammonia from ammonia slip of the first catalytic converter.
Guidelines for onboard diagnosis (OBD) require that both SCR catalytic converters must be monitored. For this purpose, as a rule a respective nitrogen oxide sensor is provided downstream of both SCR catalytic converters. For cost reasons, usually only one dispensing valve is installed upstream of the first SCR catalytic converter in order to introduce an ammonia-releasing solution of a reduction agent into the exhaust system. Filling the second SCR catalytic converter with ammonia thus takes place only by ammonia slip of the first SCR catalytic converter. The data of said sensors can be used for modeling the level of the two SCR catalytic converters. However, in the event of deviations from the modelled aging of the SCR catalytic converters, the physical levels can deviate significantly from the modelled levels. This can lead to changes in the efficiency of the nitrogen oxide reduction and thereby possibly to exceeding the emission limits.